Background of the Invention
[0001] Shear thickening fluids (STFs) are fluids whose viscosity increases with shear rate.
Of particular interest are discontinuous STFs, which at high shear rates transform
into a material with solid-like properties. A typical example of a discontinuous STF
is a stabilized suspension of rigid colloidal particles with a high loading fraction
of particles. Such systems have been studied for many different combinations of fluid
matrix and particle size and compositions (
Egres, R.G., Lee, Y.S., Kirkwood, J.E., Kirkwood, K.M., Wetzel, E.D., and Wagner,
N.J., "Novel flexible body armor utilizing shear thickening fluid composites." Proceedings
of 14th International Conference on Composite Materials. San Diego, CA. July 14-18,
2003), (
Lee, Y.S., Wagner, N.J., "Dynamic properties of shear thickening colloidal suspensions,"
Rheol Acta 42,199-208 (2003), (
Shenoy, S., Wagner, N.J., Bender, J.W., "E-FiRST: Electric field responsive shear
thickening fluids," Rheo Acta 42,287-294 (2003)). The shear thickening in the colloidal suspension is due to the formation of jamming
clusters, or hydroclusters,
Lee, Y.S., Wagner, N.J., "Dynamic properties of shear thickening colloidal suspensions,"
Rheol Acta 42,199-208 (2003) bound together by hydrodynamic lubrication forces. The hydrocluster growth and collision
eventually result in a percolated arrangement of the rigid particles across macroscopic
dimension. This microstructural transformation leads to the bulk solid-like behavior.
Upon relaxation of the applied stresses, the shear-thickened material typically relaxes
to the low strain rate, fluid-like behavior (
Eric D. Wetzel, Y. S. Lee, R. G. Egres, K. M. Kirkwood, J. E. Kirkwood, and N. J.
Wagner, "The Effect of Rheological Parameters on the Ballistic Properties of Shear
Thickening Fluid (STF) KEVLAR®Composites" NUMIFORM, 2004).
[0002] Shear-thickening fluids have been shown to have utility in the fabrication of energy
dissipative devices, such as shock absorbers (
Hesse, H., US Pat. No. 4,503,952), (
Rosenberg, B.L., US Pat. No. 3,833,952), (
Sheshimo, K., US Pat. No. 4,759,428) and more recently in the fabrication of ballistic fabric composites (
Egres, R.G., Lee, Y.S., Kirkwood, J.E., Kirkwood, K.M., Wetzel, E.D., and Wagner,
N.J., "Novel flexible body armor utilizing shear thickening fluid composities." Proceedings
of 14th International Conference on Composite Materials. San Diego, CA. July 14-18,
2003), (
Lee, Y. S., Wetzel, E. D., and Wagner, N. J., "The ballistic impact characteristics
of KEVLAR® woven fabrics impregnated with a colloidal shear thickening fluid", J.
Mat. Sci. 38, 2825-2833 (2003), (
Eric D. Wetzel, Y. S. Lee, R. G. Egres, K. M. Kirkwood, J. E. Kirkwood, and N. J.
Wagner, "The Effect of Rheological Parameters on the Ballistic Properties of Shear
Thickening Fluid (STF) KEVLAR® Composites" NUMIFORM, 2004) and puncture resistant materials ("
Stab resistance of shear thickening fluid (STF)-treated fabrics" by M. J. Decker,
C. J. Halbach, C. H. Nam, N. J. Wagner and E. D. Wetzel, accepted for publication
in Composites Science and Technology, August, 2006). There is considerable interest in incorporating STF's into other materials.
PCT/US2004/015813 entitled "Advanced Body Armor using a shear thickening fluid" is incorporated by
reference in its entirety for all useful purposes. Incorporation of STF's into plastics,
rubbers and foams is discussed below. Shear thickening fluids may also contain fillers,
see
PCT application no. US06/04581 filed February 9, 2006, which is incorporated by reference in its entirety for all useful purposes.
[0003] Within the scope of this invention, the shear thickening fluid is defined as any
fluid that exhibits an increase in viscosity with increasing shear rate or applied
stress. Shear thickening is not shear dilatancy, which is a material property whereby
the material's volume changes upon an applied stress or deformation. Shear thickening
fluids, however, may exhibit dilatancy under specific conditions.
[0004] The shear thickening fluids may be concentrated dispersions of particulates within
a fluid medium that exhibit an increase in viscosity with increasing applied stress,
the particles within the fluid would preferably have a smallest dimension being less
than 10 microns, more preferably, less than 1 micron, as well as true nanoparticles
being below 100 nm in smallest dimension. Particles can be of any solid material,
including spherical amorphous silica such as that produced via Stöber type synthesis,
synthetic inorganic particles synthesized via solution precipitation processes such
as precipitated calcium carbonate, or synthesized by gel-sol techniques (hematite,
TiO2), or fumed silica, or carbon black. Natural inorganic particulates such as montmorillonite
and kaolin clays can be dispersed in solvents and have been shown to exhibit shear
thickening behavior. Ground mineral powders, such as quartz, calcite, talcs, gypsum,
mica can be dispersed in liquid mediums and exhibit shear thickening behavior. The
solid dispersed phase can also be polymeric in nature, such as plastisols generated
through emulsion polymerization processes such as poly(methyl methacrylate) (PMMA),
polystyrene (PS) microspheres such as those available from Polysciences or Bangs Laboratories,
Inc. The solid phase can also be starch or other natural polymers.
[0005] There is significant interest in understanding the flow properties and microstructure
formation of immiscible blends of liquids. Indeed, the processing and flow of immiscible
blends, emulsions, and miniemulsions can affect the microstructure, which can in turn,
affect the final material properties. Such blends, emulsions, and miniemulsions occur
often in many systems and processes of practical importance. For example immiscible
fluids are often used in direct mass contacting operation, such as extraction, while
many foods, cosmetics, liquid soaps, and other consumer products are delivered or
processed as mixtures of immiscible fluids. Immiscible polymer blends are formulated
to achieve physical properties between the limits of the pure components, as well
as imparting new characteristics arising from the presence of an interface.
[0007] The general realization of an emulsion, such as the ones under study, is treatment
of one phase as continuous and the second as discrete droplets, either ellipsoidal
or spherical in shape, or as both phases in a continuous form (known as co-continuous).
The discrete phase droplets can deform, break up, aggregate, and coalesce under flow.
Flow induced deformation is quite complex (
Edwards, B.J., Dressler, M., "A rheological model with constant approximate volume
for immiscible blends of ellipsoidal droplets," Rheol. Acta 42, 326-337 (2003)), (
Wetzel, E.D., Tucker, C.L., "Droplet deformation in dispersions with unequal viscosities
and zero interfacial tension," J. Fluid Mech. 426, 199-228 (2001)). A more in-depth discussion of droplet deformation can be found in
Cavallo, Dagreou, S., Allal, A., Marin, G., Mendiboure, B., "Linear viscoelastic properties
of emulsions and suspensions with thermodynamic and hydrodynamic interactions," Rheo
Acta 41, 500-513 (2003), Edwards, or Wetzel (
Wetzel, E.D., Tucker, C.L., "Droplet deformation in dispersions with unequal viscosities
and zero interfacial tension," J. Fluid Mech. 426, 199-228 (2001),
Wetzel, E.D., Tucker, C.L., "Microstructural evolution during complex laminar flow
of liquid-liquid dispersions," J. Non-Newtonian Fluid Mech. 101, 21-41 (2001)). Even more important to the current work are the results of Kemick and Wagner (
Kernick, W., Wagner, N.J., "The role of liquid-crystalline polymer rheology on the
evolving morphology of immiscible blends containing liquid-crystalline polymers,"
J. Rheol. 43(3), 521-549 (1999)) showing that non-Newtonian fluids in emulsion can lead to dramatic changes in morphology
under flow due to shear rate dependence of morphology.
[0008] The analysis of the emulsion under study is performed with the premise that the state
of the discrete phase droplets can be understood through consideration of droplet
breakup and coalescence (
Doi, M., Ohta, T., "Dynamics and rheology of complex interfaces. I," J. Chem. Phys.
95, 1242-1248 (1991)), (
Taylor, G. I., "The visocity of a fluid containing small drops of another fluid,"
Proc. R. Soc. London Ser. A 138, 41-48 (1932)), (
Taylor, G.I., "The formation of emulsions in definable fields of flow," Proc. R. Soc.
London Ser. A 146, 501-523 (1934)). Work by Rusu (
Rusu, D., Peuvrel-Disdier, E., "In situ characterization by small angle light scattering
of the shear-induced coalescence mechanisms in immiscible polymer blends," J. Rheol.
43(6), 1391-1409 (1999)) highlights in detail the complexities of the dynamic equilibrium between droplet
coalescence and dispersion mechanisms. The droplet coalescence is broken down into
four stages (i) the collision between the two drops; (ii) the drainage of the matrix
film separating the colliding drops; (iii) the rupture of the matrix film; and (iv)
the drop coalescence. The work on the shear-induced coalescence of droplets is considered
to be consistent with previously understood mechanisms. However, the prediction of
final morphology (particle size distribution and state of dispersion), in connection
with the magnitude of deformation and stresses, is very difficult (
Iza, M., Bousmina, M., "Nonlinear rheology of immiscible polymer blends: Step strain
experiments," J. Rheol. 44(6), 1363-1384 (2000)).
[0009] Adding more of the dispersed phase can lead to the formation of a co-continuous morphology
or phase inversion.
Ageropoulos, G.N., Weissert, F.C., Biddison, P.H., Bohm, G.A., "Heterogeneous blends
of polymers. Rheology and Morphology," Rubber Chem. Technol. 49, 93-104 (1976) concluded that the point of phase inversion is reached when the torque ratio of
the components is equal to the component volume fraction ratio. Work by Utracki (
Utracki, L.A., "On the viscosity-concentration dependence of immiscible polymer blends,"
J. Rheol 35(8), 1615-1637 (1991)) suggests the phase inversion point can be predicted based on the dependence of
the viscosity on the volume fraction of monodispersed hard spheres in the matrix,
as proposed by Krieger and Dougherty (
Krieger, I.M., Dougherty, T.J., "A mechanism for non-Newtonian flow in suspensions
of rigid spheres," Trans. Soc. Theol. 3, 137-152 (1959)). A more in depth analysis of the previous work in the area of phase inversion can
be found in work by
Astruc, M., Navard, P., "A flow-induced phase inversion in immiscible polymer blends
containing a liquid-crystalline polymer studied by in situ optical microscopy," J.
Rheol. 44(4) 693-712 (2000).
[0010] The blend or emulsion under study is a mixture of a shear thickening fluid comprised
of particles in a suspending media, dispersed in an immiscible liquid (Figure 1).
The solid particles are suspended in the suspending media at a sufficient loading
to produce a shear thickening fluid. The particles are not added to the silicone phase
and the emulsion can be reduced to two phases. Such realizations are known as
suspoemulsions, which refer to the dispersion of a suspension in an immiscible liquid. Phase inversion,
either by concentration or processing, can lead to the dispersion of the immiscible
liquid into the shear thickening fluid, or the existence of co-continuous microstructure.
[0011] Previous literature is limited to a few investigations dealing with a shear thickening
phase in an emulsion (
Pal, R., "Non-idealities in the rheological behavior of emulsions," Chem. Eng. Comm.
121, 81-97 (1993)), (
Tan, H., Tam, K.C., Jenkins, R.D., "Rheological Properties of Semidilute hydrophobically
Modified Alkali-Soluble Emulsion Polymers in Sodium Dodecyl Sulfate and Salt Solutions."
Langmuir 16, 5600-5606 (2000). Pal (
Pal, R., "Non-idealities in the rheological behavior of emulsions," Chem. Eng. Comm.
121, 81-97 (1993)) studied the rheological behavior of pure component EDM oil, a non-discontinuously
shear thickening fluid, and emulsions containing EDM. Results showed that with the
addition of an emulsifying agent the oil could be emulsified in deionized water (Newtonian
fluid) resulting in an emulsion with Newtonian viscosity behavior.
[0012] Studying the rheology of immiscible dispersions of a non-Newtonian liquid-crystalline
polymer Kemick and Wagner found dramatic changes in morphology under flow due to shear
rate dependence of morphology. It is therefore expected that a suspoemulsion containing
the highly non-Newtonian shear thickening fluid will have a change in morphology under
different flow conditions.
[0013] Such suspoemulsions can have useful applications in and of themselves due to their
complex rheological properties. In addition, such suspoemulsions can be precursors
to forming STF composite materials that can have useful applications. An example of
the latter is a polymer blend containing STF inclusions, which is processed when the
polymer is above or near to its melting transition such that it can be processed,
but is used below this temperature where the polymer is more rigid. Another example
is the formation of a suspoemulsion in a crosslinkable or polymerizable matrix fluid
such that the STF composite is formed upon crosslinking or polymerization of the matrix
fluid.
Summary Of The Invention
[0014] One aspect of the invention is drawn to a composite which comprises a shear thickening
fluid (STF) combined with an immiscible or partially miscible matrix material wherein
the STF comprises a solvent and particles suspended in said solvent and the particles
are colloidal particles having a particle size less than about 10 microns.
[0015] Another aspect of the invention is drawn to a composite which comprises a suspoemulsion,
where the suspension in the suspoemulsion is a shear thickening fluid.
[0016] A further aspect of the invention is drawn to an immiscible blend of a polymer which
comprises a polymer with an immiscible or partially miscible shear thickening fluid
(STF).
[0017] A further aspect of the invention is drawn to a composite which comprises a shear
thickening fluid (STF) in a foam.
Brief Description Of The Figures
[0018]
Figure 1 is a pictorial representation of emulsion according to the invention with
shear thickening fluid as discrete phase.
Figure 2a illustrates droplets of silicone in shear thickening fluid, ΦSTF= 63.4. Figure 2b illustrates droplets of shear thickening fluid in silicone oil,
ΦSTF=10.4
Figure 3 illustrates the Viscosity-Shear rate experimental results for the STF, neat
silicone fluid, and suspoemulsions.
Figure 4a illustrates silicone droplets under shear, ΦSTF=63.4. Figure 4b illustrates silicone in discrete phase shown at rest, ΦSTF=63.4. Figure 4c illustrates silicone in discrete phase under shear, ΦSTF=63.4, displaying droplet deformation.
Figure 5 illustrates time lapse photographs of sample subjected to shear and allowed
to relax.
Figure 6 illustrates stress-time droplet evolution.
Figure 7a illustrates the discrete regions of STF formed in the silicone can be observed
in a sample 18% mass STF. Figure 7b shows the ribs of shear thickening fluid formed
in Silastic T2 (28% mass STF), highlighted with an arrow, upon curing. Figure 7c shows
a large droplet of shear thickening fluid (white area) that was contained inside of
a cured Silastic T2 silicone sample.
Figure 8 shows the results of compression testing of neat Foam and STF-foam composites
for various rates of deformation.
Figure 9 shows micrographs of the phase morphology in STF polymer composites obtained
by scanning electron microscopy.
Figure 10 shows ASTM Trouser Tear Test Results for STF-PO samples.
Detailed Description Of The Invention
[0019] We show novel materials formed from shear thickening fluids in scaffolds, such as
foams. We determined the rheological properties of an emulsion containing a shear
thickening fluid immiscibly blended with silicone oil, forming a novel shear thickening
suspoemulsion. The system is also hypothesized to follow standard shear droplet morphology,
with competition between coalescence and breakup. We also formed such shear thickening
suspoemulsions using STFs blended with melted polymer, such that upon cooling, the
novel microstructures formed during processing were "frozen" in upon solidification.
The latter yielded materials comprising encapsulated shear thickening fluids with
greatly enhanced material properties.
Solid-shear thickening fluid composites
[0020] One method comprises STFs in scaffold materials. Such scaffold materials would include,
but are not limited to, open cell foams, such as those produced from polyurethanes,
polyolefins or any polymeric material. Another porous scaffold material that would
have utility is expanded polytetrafluoroethylene (ePTFE) such as found in several
materials sold under the trade name GORE-TEX® (W. L. Gore and Associates, Inc.), or
microporous or expanded polyethylene. Another scaffold material would be an engineered
porous plastic or rubber, where inclusions are available to be filled partly or completely
with STF.
[0021] Another type of scaffold material would include porous natural materials, including
but not limited to wood, porous stone such as sandstone, pumice etc., and the skeletons
of marine organisms including corals and sponges.
[0022] Alternatively, the scaffold material could be a building material such as concrete
or asphalt. The presence of shear thickening fluid could absorb damaging impact energy
within these materials which contribute to fatigue failure or catastrophic failure
of articles made therefrom.
[0023] A second method by which a rubbery or solid (referred to herein as the second material)-shear
thickening fluid composite containing discrete or co-continuous regions of shear thickening
fluids, such as a suspoemulsion, can be fabricated is via mixing STF with an immiscible
or partially immiscible liquid or fluid-like component (such as a molten polymer,
or resin that can be foamed or crosslinked) that can subsequently be converted to
a solid through heating, cooling, chemical reaction, etc. Candidate materials would
preferably exhibit little solubility with the liquid suspending medium used to generate
the shear thickening fluid, at least on the time scales and at the processing conditions
required to transition the material from a liquid to a solid. Control of the morphology
can be tuned based on choice of shear thickening fluid and second material liquid
component, the relative compositions of the STF and second material liquid component,
shear or processing conditions, and the addition of process adjuvants such as surfactants,
fillers, block copolymers, nano and colloidal particles, such as used in pickering
emulsions, and the like.
[0024] One type of second material would include reactive polymeric materials that cure
or crosslink to form solids. Reactive polymers include polyurethanes that cure through
the chemical reaction of components (polyols and isocyanates), epoxies that cure through
the addition of a catalyst, and UV curable resins. A preferred second material of
this type would be from the class of elastomeric or elastomeric gel materials, such
as silicone rubber (cross-linked PDMS) or silicone gels and the like, which can be
relatively low viscosity liquids prior to cross-linking, whereafter they form resilient
materials with good rebound characteristics. A variety of elastomers exist to provide
a wide range of properties such as chemical and solvent resistance, temperature resistance,
and hardness (durometer). These materials could be mixed with shear thickening fluids
at room temperature to disperse the shear thickening fluids adequately and to achieve
the desired composite morphology or shear thickening fluid droplet size. The liquid-like
second material could subsequently be cured, or the curing could be accelerated through
heating or the addition of additional components that catalyze the reaction and transform
the second material into a solid.
[0025] Another type of second materials would include melt processable polymers or thermoplastic
elastomers (TPE). Melt processable polymers include but are not limited to polyolefins
such as polyethylene and polypropylene, nylon, polymethylmethacrylate, polyvinylchloride,
polyethylene terephthalate (PET), polycarbonate and the like. Thermoplastic elastomers
would include such as materials as those sold under the trade names Santoprene™ (Exxon
Mobil Chemical), Hytrel® (DuPont Company), and Engage™ from DuPont-Dow Elastomers.
In this instance, increased temperature is used to liquefy a polymeric material. At
the processing conditions required to achieve the desired melt flow properties of
the polymer second material, the shear thickening fluid would be compounded with the
polymer melt to achieve the desired level of mixing and microstructure. The temperature
would subsequently be reduced to generate the solid polymer-shear thickening fluid
composite.
[0026] End use articles can be generated from the aforementioned novel materials through
injection molding, thermoforming, extrusion, and conceivably any method used to fabricate
articles from the second polymeric material alone as known to those skilled in the
art.
[0027] Alternatively these materials could be incorporated as discrete laminar regions between
layers of the second material, such as between layers of polymeric materials and composites
(such as polycarbonate and fiberglass reinforced epoxy) glass, ceramic, or metal films
or sheet. Such laminar composites could exhibit dramatically improved ballistic or
impact related energy dissipation, as the compression associated with impact would
initiate improved energy dissipation between the second material layers within the
construction. The particles in the shear thickening fluid of the invention are preferably
colloidal particles having a particle size less than 10 microns and more preferably
less than one micron. The laminates formed are different than the laminates disclosed
in
WO 2004/012934 (WO '934). The laminates disclosed in WO '934 require a specific shape (corrugated
egg crate shape). The invention can be practiced not using the specific shape required
by WO '934.
[0028] The shear thickening composite materials are different than those disclosed by Plant
(
US 2004/0171321) as they describe a dilatant material impregnated into a resilient carrier. The dilatant
material are dilatant compounds and putty and are distinct from the shear thickening
fluids described in this work.
[0029] Liquid-shear thickening fluid composites: One embodiment of the invention relates to a composite liquid-liquid mixture, or
blend, where the two liquids are immiscible or only partially miscible such that they
develop a heterogeneous morphology, and where at least one component in the mixture
is a fluid that exhibits a shear thickening transition, (increase in viscosity resulting
from increased shear stress or shear rate). We have shown that blends of shear thickening
fluids incorporated into a second fluid medium exhibit rheological behavior indicating
their ability to impart improved energy dissipative capabilities to composites made
there from.
[0030] Such a fluid may have utility in control and damping applications. One such application
may be a mechanical limiting fluid that resists deformation beyond a certain applied
stress or deformation rate. Such a fluid may have utility when incorporated as a coating,
impregnated into or dispersed within a solid material. Such a fluid could serve as
a layer between two materials such as glass, metals, or plastics to provide energy
dissipative characteristics. One application for such a fluid would be to impregnate
or imbibe it into a porous solid scaffold material. Such solid material structures
would include, but are not limited to fiber yarns, woven fabric, spunlaced or spunbonded
nonwoven fabric, an open cell foam (to form a co-continuous network with the solid
open cell foam network/scaffold), or to exist as discrete droplets within a solid.
The composition of the yarns or fabrics would include synthetic materials, such as
polymers, elastomers, as well as inorganic fibrous materials. Polymeric fibers include
high modulus polymeric fibers such as polyaramids poly(phenylene diamine terephthalate)
sold by Dupont under the registered trademark KEVLAR®, nylon, polyester, Dacron etc.
Elastomeric fibers include polyisoprene, other fibers included within the scope of
this invention include high modulus inorganic fibers and/or fiber yarns and/or fabrics,
or natural occurring fibers such as cotton, wool, hemp. etc.
High modulus inorganic fibers and fiber yarns and fabrics include, but are not limited
to graphite, E-glass, S-glass, ceramics fibers etc.
[0031] The novel blend could be incorporated as discrete droplets or co-continuous phases
within a solid material by methods described earlier for fabricating solid-shear thickening
fluid composites.
Distinction between our invention and dilatant energy absorbing materials
[0032] Palmer et al. [US 2005/0037189 A1,
WO 03/055229 A2,
WO 2005/000966 A1] describe an invention whereby a "polymer-based dilatant" is distributed into a solid,
foamed synthetic polymer matrix to form a "self- supporting energy absorbing composite".
Plant (
US 2004/0171321) describes a dilatant material impregnated into a resilient carrier. These applications
use a known, commercially available polymer (i.e., "silly putty") that dilates under
strong deformation as a component in a composite material. These applications are
limited in scope and are fundamentally different than what is proposed here, in that:
A)
dilatant materials expand in volume when subject to stress, while
shear thickening materials are essentially incompressible and have a viscosity that increases with
shear stress; B) the shear thickening materials consist of colloidal and nanoparticle
dispersions, as opposed to " a material in which the dilatancy is provided by polymer
alone or by a combination of polymer together with one or more other components, e.g.
finely divided particulate material, viscous fluid, plasticizer, extender or mixtures
thereof, and wherein the polymer is the principle component" [
Palmer and Green, US 2005/0037189 A1,. Thus, the invention described herein is distinguished from the work of Palmer et
al. and Plant by both principle and materials, and is not derivative thereof.
Applications
[0033] Applications for shear thickening fluids incorporated into pads for sports equipment.
Bicycle, motorcycle helmet, construction hardhat and the like which incorporates padded
regions between the outer shell and the wearers head containing shear thickening fluids
either in suspoemulsions, impregnated in foams, existing as discrete droplets or stripes
or layers within an elastomer, or as part of a co-continuous or interpenetrating network
of shear thickening fluid network within a solid plastic or elastomeric material.
[0034] A mouth guard fabricated from a conformable polymer or elastomer containing discrete
or co-continuous regions of shear thickening fluids would likely exhibit improved
energy dissipation. Such an article could reduce the likelihood of concussion, dental
damage, etc.
[0035] Conformable Pads incorporating discrete regions, dispersed droplets, co-continuous
networks of shear thickening fluids within a solid polymeric or elastomeric second
material could be incorporated into gloves that can serve to reduce vibration or protect
the hands from a jarring impact. One possible application would be the wrist protection
guards worn during roller skating and roller blading. Additionally, such conformable
padding could be used to generate knee pads, elbow pads and the like for providing
improved safety from impact to specific regions. The padding can be used in any other
sport that requires padding, such as but not limited to baseball, football, hockey,
lacrosse, fencing. Besides gloves, elbow and knee pads, other examples of uses of
the padding include but are not limited to shin guards, shoulder pads, leg pads, chest
protectors, and athletic cups.
[0036] A conformable cast material can be fabricated from the polymeric or elastomeric-shear
thickening fluid composites containing discrete droplets, or co-continuous regions
of shear thickening fluids. Such materials could provide improved protection by limiting
movement (twisting, bending) of a broken bone, muscle strain, sprains, or injured
limb (wrist, elbow, ankle, knee, neck, spine or the like. Such a material could also
be used as a cast material worn while strains and sprains are healing similar to air
casts to the ankle, wrist or the like which allows movement but protects against rapid
twisting or impacts which could re-injure the region. Such materials can be of application
in orthopedic applications, where protection from fall and impact of hips, knee and
elbow and other joints is desired.
[0037] The novel conformable composite comprised of discrete droplets, or co-continuous
networks of shear thickening fluids could be used in seat cushioning and neck supports
in automobiles, airplanes, trains and the like to provide more protection during accidents.
This would be particularly beneficial for protecting the spine and neck from injury
such as from torque or whiplash.
[0038] Pads fabricated of conformable and resilient composites containing shear thickening
fluids as discrete droplets or co-continuous networks within a plastic or elastomer
second material could be positioned at high stress and/or frequently impacted regions
such as beneath the heel and beneath the ball of the foot. The nature of the fluid
within these types of pads could also be a large pocket of fluid directly local to
the point of impact or potential point of impact. These pads could be incorporated
directly into a sports shoe design or as part of a shoe insert to reduce impact during
walking, running, jumping and the like. The pads can be used as an orthotic placed
in the shoe. A potential energy dissipative construction for an athletic shoe would
allow shear thickening fluid to pass through channels connecting pockets located beneath
the ball of the foot and heel, allowing transfer of shear thickening fluid between
both impact regions as a result of the natural heel-to-toe weight transfer associated
with walking, running and the like. Such materials can be of military application
in boots such as but not limited to paratrooper boots.
[0039] Mechanical components could be fabricated from solid materials containing shear thickening
fluids as discrete droplets or regions or as a co-continuous network within the solid
material. Smart components could be fabricated where the stiffness or hardness of
a flexible component can change as a result of degree of deformation, such as from
elongation, bending, torque, twisting, compression, or dependent on the rate of elongation,
bending, torque, twisting, compression. Such components could have utility as part
of a suspension which could stiffen in response to bending, etc.
[0040] The novel blends of shear thickening fluids and a second liquid material discussed
earlier could have utility in several applications used as a medium to control mechanical
actuation of one object relative to another. This material could be used as a shock
absorbing medium, such as within the rotary shock absorber discussed by
Hesse, US Pat. 4,503,952, the viscoelastic damper discussed by
Seshimo (US pat. 4,759,428), or the tug resistant link discussed by
McMahon et al. (US Pat. 5,712,011). Additionally, the novel energy dissipative blend could be used to control, attenuate,
or facilitate the transfer of energy between two or more objects such as that associated
with torque, translation and vibration. These novel composites could be used in hard
drive and CD drive vibration or shock dissipation, or to protect from impact sensitive
mechanical, electrical, or optical equipment.
Examples:
Example 1, Shear Thickening Suspoemulsion
[0041] Figure 1 illustrates a representation of the suspoemulsions with shear thickening
fluid (STF) as the dispersed phase
STF-Suspoemulsion Preparation: The first part of the two-part emulsion is a shear thickening fluid (STF) which comprises
polyethylene glycol and silica particles. The fluid was prepared using amorphous silica
powder with an average particle diameter of 450 nm (Nippon Shokubai KE-P50) dispersed
in 200 MW Polyethylene Glycol (Clarient PEG-200), Table 1. The silica particles were
first added to PEG at a weight fraction of 30% with the diluteness of the system aiding
in particle dispersion. This mixture was then centrifuged for 2 hours and the supernatant
removed, leaving a cake of close packed silica particles (Φ∼64%). Very small amounts
of PEG were added to the silica particles until the mixture reached the desired flow
properties, higher additions of PEG at this stage would decrease the viscosity of
the solution. PEG can be easily replaced by lower molecular weight solvents, such
as ethylene glycol. Thermogravimetric analysis was conducted on the sample to determine
the weight fraction of particles in this final solution, 59.52%, which was then used
to determine the volume fraction of silica in PEG. All emulsions were formulated with
the same 49 vol % silica shear thickening fluid.
Table 1 : Material Properties
|
p (g/cm3) |
ηo (Pa-s) |
Silica |
2.055 |
- |
PEG |
1.02 |
0.057 |
Silicone Part A |
1.12 |
21.4 |
[0042] The second part was a silicone oil, acquired from GE Silicones (SLE5700-D1), and
was chosen for its Newtonian viscosity behavior and high immiscibility in PEG. For
the rheological testing only Part A of the two part polymerizable silicone was used.
The silicone can be polymerized with the addition of the catalyzing agent, Part B.
[0043] The emulsions were prepared by weighing out a specific mass of each fluid in a 20mL
glass scintillation vial and then mixing by hand with a spatula. The samples studied
are given in Table 2, with a wide compositional range. The samples will be referenced
by the volume fraction of shear thickening fluid contained, Φ
STF.
Table 2: Emulsion Compositions
Mass of Shear Thickening Fluid (gm) |
Mass of Silicone (gm) |
Weight % STF |
ΦSTF |
3.042 |
0.379 |
88.9 |
82.9 |
2.2796 |
0.7922 |
74.2 |
63.4 |
1.721 |
0.809 |
68.0 |
56.2 |
3.904 |
3.915 |
49.9 |
37.5 |
1.938 |
5.946 |
24.6 |
16.4 |
0.7048 |
3.6701 |
16.1 |
10.4 |
[0044] STF-Suspoemulsion Structure: The two components were mixed by hand and a sample of this emulsion plated onto a
glass slide for inspection under an optical microscope; representative images are
shown in Figures 2a and b. It was suspected that the absence of a mechanical mixer
would create a very polydisperse droplet size due to the non-uniform power input (
Kitade, S., Ichikawa, A., Imura, N., Takahashi, Y., Noda, I., "Rheological properties
and domain structures of immiscible polymer blends under steady and oscillatory shear
flows," J. Rheol. 41(5), 1039-1060 (1997)). Visual confirmation of the formation of two distinct phases enabled sizing of
the disperse phase. The droplet size of the disperse phase as prepared was found to
range between∼1 and 100 µm.
[0045] STF-Suspoemulsion Rheology: A 25mm cone and plate geometry (gap=0.0509 mm, angle=0.099 radians) was utilized
on a strain-controlled rheometer (Rheometrics SR-5000) to determine the rheological
properties of each emulsion and the pure components. Testing was performed at a constant
25 °C with a peltier and constant temperature water bath used for temperature control.
The samples were subjected to a uniform force gap testing followed by a creep test
at 5 Pa until the viscosity of the sample reached a steady state, approximately two
minutes. This was done to insure homogeneity of the sample and to set a fixed shear
history for each test. This constant preshear should result in an emulsion morphology
at steady state as a balance between droplet breakup and coalescence. Stress sweeps
were run on the rheometer with a maximum time of 30 seconds per data point and frequency
sweeps were run without a time limitation for each data point. After completion of
the rheological tests a portion of the used sample was then plated for observation
under the optical microscope. The quick relaxation time of the sample made quantitative
analysis of samples plated after testing in the rheometer inaccurate, as the system
relaxed before the phase microstructure could be analyzed. The measurements, however,
helped in qualitative understanding of the effects of shear flow on the sample morphology.
[0046] A simple shear cell was constructed by inserting sample between two glass microscope
slides. The slides could then be sheared with respect to each other and the resulting
fluid behavior observed under an optical microscope with image capturing software.
[0047] A more complete analysis of the shear thickening phenomenon for systems very similar
to the one studied is given by Lee and Wagner (
Lee, Y.S., Wagner, N.J., "Dynamic properties of shear thickening colloidal suspensions,"
Rheol Acta 42,199-208 (2003)) and the interested reader is referred to that manuscript for a thorough explanation
of STF rheological behavior. Barnes (
Barnes, H., J. Rheol. Vol. 33, 329 (1989)) provides an extensive, but not exhaustive, list of possible shear thickening fluids.
[0048] The viscosity as a function of both strain rate and stress for the neat materials
and suspoemulsions are shown in Figure 3.
[0049] As can be seen the viscosity of the emulsion is highly dependent on the volume fraction
of shear thickening fluid in the mixture and the shear stress imparted. The viscosity
of each emulsion is a nontrivial combination of the pure component curves. At low
shear thickening fluid volume fractions, the samples display Newtonian like fluid
behavior at low stresses and a very small shear thinning region. At higher STF loadings
the low shear viscosity more closely tracks that of the STF. The emulsions with the
highest volume fractions of shear thickening fluid (Φ
STF=82.9, 63.4, 56.2) also exhibit discontinuous shear thickening at increasingly higher
stress levels while the lower volume samples exhibit shear thickening followed by
a plateau and then a second shear thinning region.
[0050] The critical stress for shear thickening is defined as the stress at the minimum
in the viscosity vs. shear stress curve at the onset of shear thickening, and is shown
in Table 3.
Table 3 Critical Stress Values for Shear Thickening as a Function of STF Phase Volume
ΦSTF |
Critical Rate (S-1) |
Critical Stress (Pa) |
Critical Viscosity (Pa-s) |
100 |
3.583 |
43 |
12.1 |
82.9 |
1.777 |
40 |
22.5 |
63.4 |
4.647 |
49 |
10.4 |
56.2 |
5.095 |
56 |
11.0 |
37.5 |
2.576 |
126 |
49.1 |
37.5 |
3.554 |
158 |
44.5 |
16.4 |
7.633 |
251 |
32.8 |
10.4 |
6.297 |
186 |
29.5 |
[0051] This is clear evidence that the STF phase in the novel STF suspoemulsions is able
to exhibit shear thickening. The stress required for shear thickening is highest at
low STF loadings.
[0052] Thixotropy in the form of hysteresis in the flow curves was observed at intermediate
and low STF phase volumes. This is described in greater detail in
U.S. application Serial Number 11/260,742 filed October 27, 2005 which again is incorporated by reference in its entirety including all the figures.
Such hysteresis is a sign of shear sensitivity of the microstructure.
[0053] The viscosity-concentration relationship of the mixtures was determined by taking
the viscosity of each component at constant stress values. This relationship is shown
in Table 4

[0054] Once again a behavioral difference is seen as the volume fraction of shear thickening
fluid is increased. At low STF loadings the effects of shear stress on viscosity are
minimal, whereas at higher STF loadings, the nonlinear dependence of the STF component
is evident in the emulsion viscosity.
[0055] Dynamic tests were run for all samples at both one and five radians per second so
that the linear viscoelastic region could be determined. This is described in greater
detail in
U.S. application Serial Number 11/260,742 filed October 27, 2005, which again is incorporated by reference in its entirety including all the figures..
The low STF loading samples show minimal effects of increasing stress amplitudes,
whereas the high STF loading emulsions show non-linear behavior typical for the STF
component (
Lee, Y.S., Wagner, N.J., "Dynamic properties of shear thickening colloidal suspensions,"
Rheol Acta 42,199-208 (2003)). There are seemingly few differences between the responses of the mixtures at the
two different frequencies tested. The viscous contribution is seen to be an order
of magnitude greater at the high frequency where as the elastic contribution is on
the same order for both frequencies.
STF-Suspoemulsion Structure by Optical Microscopy:The samples were imaged to determine the continuous and discrete phases formed in
emulsions with varied shear thickening fluid compositions. As shown in Figures 2a
and 2b it was possible to create discrete phases of each component in a continuous
phase of the other. An analysis of the response of the emulsions to shear was performed
using the shear cell constructed of glass slides. A small amount of sample was plated
between two glass slides allowing shear of the sample by movement of either slide
while fixing the other. The shearing between plates was done by hand. The setup permits
one to view in a direction perpendicular to the flow field.
[0056] Figure 4a shows a sample with Φ
STF=63.4 under a stress that is not known, an arrow is added to indicate the direction
of shear. What can be seen is the elongation of the silicone droplets under this shearing
action. Figure 4b shows this same sample at rest and again under shear in 4c, establishing
that the STF inclusions remain in a flowable form inside the suspoemulsion.
[0057] To further investigate whether the STF suspoemulsion can relax after deformation
and shear thickening, a sample consisting mostly of shear thickening fluid, Φ
STF=63.4, was plated on top of a single glass slide (no cover) and subjected to a shear
stress at t < 0. Once the stress was removed the state of the system was recorded
using image-capturing software on the microscope. This is shown in Figure 5, with
silicone occupying the discrete regions in a continuous shear thickening fluid matrix.
The time stamps of when the pictures were taken are given and could be used to calculate
the surface tension between the phases. Evident from the slides is the elongation
of the discrete phases of silicone and the subsequent relaxation back into a spherical
shape. Some of the elongated threads are observed to break into droplets. This overall
behavior is expected as a lower energy state is reached with a decreased interfacial
area and is in agreement with the predictions of
Edwards, B.J., Dressler, M., "A rheological model with constant approximate volume
for immiscible blends of ellipsoidal droplets," Rheol. Acta 42, 326-337 (2003).
STF Suspoemulsion Discussion of Results
[0058] The rheological behavior of the emulsions is interesting as discrete phases of a
shear thickening fluid are shown to influence the flow properties of a Newtonian fluid
matrix at low concentrations of shear thickening fluid. The nature of the experimental
method does not allow for visual confirmation of microstructure under shear, as would
be possible in a glass Couette cell (
Rusu, D., Peuvrel-Disdier, E., "In situ characterization by small angle light scattering
of the shear-induced coalescence mechanisms in immiscible polymer blends," J. Rheol.
43(6), 1391-1409 (1999)). However, through simple analysis of the rheological behavior and pictures of the
system at steady state an understanding of the structure and continuity of the system
can be developed. Analysis can begin keeping in mind the key results of Figures 2a
and 2b. Figure 2a shows that when small amounts of silicone are added to the shear
thickening fluid discrete regions of silicone are formed. The same is true for small
amounts of shear thickening fluid added to silicone, Figure 2b. It is then hypothesized
that there is a phase inversion between a continuous phase of shear thickening fluid
and a continuous phase of silicone at some intermediate volume fraction of shear thickening
fluid.
[0059] Figure 3 displays the viscosity stress relationships of the emulsions and clearly
shows the shear thickening behavior for each emulsion tested with flow curve characteristics
dependent on volume fraction of shear thickening fluid. The discontinuous shear thickening
observed in pure shear thickening fluid is also observed in the samples with a volume
fraction of fifty-six percent shear thickening fluid. As the amount of shear thickening
fluid decreases, the behavior changes as the fluids display a shear thickening transition
followed by a second region of shear thinning at higher shear rates, Figure 3. These
results partially confirm the hypothesis that samples with shear thickening fluid
in the continuous phase will exhibit discontinuous shear thickening while samples
with shear thickening fluid in the discrete phase will not. It is also evident that
there is a different response for emulsions around the region where shear thickening
fluid is half of the mixture by mass (Φ
STF=37.5). Also apparent is the change in shear thinning behavior for samples as the
volume of shear thickening fluid is lowered, from a very steep and drastic shear thinning
regime to a flatter, more Newtonian like fluid behavior.
[0001] The dynamic rheological testing of the emulsion can also be used to extract
behavior differences between samples. Analysis of the loss modulus shows that for
the samples with Φ
STF>37.5 there is a very steep shear thickening transition with increasing stress amplitude.
In the sample where Φ
STF≤37.5 the shear thickening response is characteristically different and subdued from
the pure shear thickening fluid. This response difference suggests that when that
sample is at rest, as in small amplitude dynamic testing, the shear thickening response
is different from the response under steady shear. The shear thickening response in
the shear testing is more apparent for the low volume (Φ
STF=16.4, 10.4) shear thickening fluid samples than in the dynamic testing where the
low volume fraction samples more closely resemble the pure silicone oil sample. This
suggests the shearing of the sample affects the microstructure of the sample. A more
in depth discussion of the dynamic properties of the shear thickening fluid response
can be found in work done by Lee and Wagner (
Lee, Y.S., Wagner, N.J., "Dynamic properties of shear thickening colloidal suspensions,"
Rheol Acta 42,199-208 (2003)).
[0060] For emulsions with a low volume fraction of shear thickening fluid, Φ
STF=10, the silicone is the continuous phase and the shear thickening fluid is dispersed
in discrete droplets. An important concept for understanding the flow field would
be a characterization of the droplet break up and coalescence, Figure 6.
[0061] It is hypothesized that the emulsion droplets follow typical behavior under the applied
shear stresses such that a dynamic equilibrium will be reached for a given steady
flow between droplet breakup and coalescence. That leads to a droplet size distribution
proportional to the power input, or applied stress, on the fluid. The higher the stress,
the smaller the average droplet becomes, whereas when the applied stress is decreased
the droplets coalesce under flow and the average size increases (
Wagner et al. Doi-Ohta model for blends AICHE J. 45 (1999)). This becomes complicated by the presence of a shear thickening fluid in the droplet
phase, as beyond a certain critical droplet stress the behavior of the STF will transfer
from a liquid to solid-like phase. At this critical droplet stress the stress of the
bulk phase is hypothesized to be the measured critical stress shown in Table 3.
[0062] At first consideration it is expected that the critical droplet stress required to
initiate the shear thickening response is equivalent for emulsions of various volume
fractions of shear thickening fluid. This would be expected as the shear thickening
response is stress controlled. For emulsions with Φ
STF of around 65% and higher the critical stress value are equivalent to that of pure
component shear thickening fluid. This supports the observation that the shear thickening
fluid is the continuous phase at high concentrations of STF. It is not known, however,
if silicone is also co-continuous at any of the volume fractions. It is remarkable
that emulsions with lower STF loadings still show a clear signature of shear thickening.
At the lowest loadings, the STF phase cannot be co-continuous and yet, a clear transition
is evident even if it does not lead to the discontinuous behavior observed for the
higher STF loadings. It is very significant that shear thickening is induced in the
droplets, as this has never been previously reported.
[0063] Evidence for the possibility of microstructure formation within the emulsion under
shear flow at high stress levels, such as a phase inversion from discrete to continuous,
is found in Table 4. The viscosity for a constant stress value changes drastically
with increasing composition. In the region of lower volume fraction the viscosity-concentration
relationship is relatively independent of stress. In the region of high shear thickening
fluid volume the relationship is more dependent on the stress. This is due to the
shear thinning behavior of these samples. The flip from a simple relationship to a
highly stress dependent one occurs when the volume fraction of shear thickening fluid
is approximately one half. This suggests a phase inversion point occurring in this
regime, presumably shifting from a continuous phase of silicone to one of shear thickening
fluid.
[0064] The dynamic frequency sweeps for each material can be used to further support our
observations about phase inversion. The crossover frequency, G'=G" (G'
c), for each sample was determined by finding the intersection of the two curves. For
samples not showing crossover within the range of the testing limits the crossover
frequency was determined by extrapolating the moduli curves to an approximate intersection.
Where there is a continuous phase of silicone oil the dominant relaxation time of
the system is close to the value of that of the pure component. Conversely, the relaxation
time of the system is close to the value of pure shear thickening fluid when the volume
fraction is higher than 50%, which supports phase inversion.
Example 2 Colloidal Shear Thickening Fluid in Foams and Cured Rubbers Composites
[0065] Sample Preparation: The shear thickening fluid (STF) was prepared using amorphous silica powder (Nippon
Shokubai KE-P50) dispersed in 200 MW Polyethylene Glycol (Clarient PEG-200). The average
diameter of the silica particle was 450 nm and the density was determined to be 2.055
g/cm
3 at 25 °C using a density meter (Anton-Paar DMA 48). The silica particles were first
added to PEG at a weight fraction of 30% with the diluteness of the system aiding
in particle dispersion. This mixture was then centrifuged for 2 hours and the supernatant
removed, leaving a cake of close packed silica particles (Φ∼64%). Very small amounts
of PEG were added to the silica particles until the mixture reached the desired flow
properties, higher additions of PEG at this stage would decrease the viscosity of
the solution. Thermogravimetric analysis was conducted on the sample to determine
the weight fraction of particles in this final solution, 59.52%, which was then used
to determine the volume fraction of silica in PEG, 49 vol%.
[0066] Four sizes of polyurethane open cell foam were used, with an average pore size of
60, 70, 80, and 90 pores per inch (ppi). They were each received from Foamex International
and have a density of ∼0.1 gm/cm
3.
[0067] A negative poisson ratio Teflon foam was acquired from W.L. Gore for testing purposes.
The material construction does not allow the shear thickening fluid to wet the foam
due to the surface tension. We anticipate improved performance using a STF that wets
the foam.
[0068] Neat samples of various silicones were cured as were samples with encapsulated regions
of shear thickening fluid. To encapsulate the shear thickening fluid the components
were massed and mixed together by hand until an emulsion of the silicone and shear
thickening fluid was generated. This emulsion was set into a mold and put in the oven
at seventy degrees Celsius until cured. The oven was used to speed the kinetics of
the polymerization.
[0069] A two-part polymerizable silicone oil, acquired from GE Silicones (SLE5700-D1), has
been rheologically characterized in Part I of this paper. The silicone can be polymerized
with the addition of the catalyzing agent, Part B, in a 50/50 weight ratio with Part
A.
[0070] Two elastic silicones were tested, Silastic T2 and Silastic S with green curing agent
(Dow Coming). Silastic T2 is a translucent high strength mold making silicone and
is created using a 10:1 1 ratio. Silastic S is also a high strength silicone with
the same mixing ratio. Both cure at room temperature and are immiscible with shear
thickening fluid.
[0071] A two part room temperature foam was acquired from Quantum Foam Resins, LDF 8, with
a mix ratio of 100:7. The density of the first part is ρ
QF_A= 1.2 gm/cm
3 and plays an important part in the suspension of the shear thickening fluid, ρ
STF=1.67 gm/cm
3. If the density ratio were too large the settling time of the shear thickening fluid
would be on a time scale less than that of the curing process. A florescent dye (DayGlo
Color Corp, D-824) was added to the shear thickening fluid to aid in visual recognition
of shear thickening inclusions. The following foam compositions were formed, Table
17.
Table 5: Closed Cell Foam Compositions, Quantum Foam Resins LDF 8
ΦSTF |
Part A (gm) |
Part B (gm) |
Mass of Shear Thickening Fluid (gm) |
0 |
4.0528 |
0.3226 |
0 |
10 |
3.5655 |
0.2427 |
0.5351 |
25 |
3.0468 |
0.2051 |
1.4597 |
50 |
2.0814 |
0.1446 |
2.8718 |
[0072] Testing: Controlled Compression: The response of the materials to a controlled compression between two parallel plates
was tested. The compression was done between two parallel plates on a rheometer (25mm
Parallel Plate, Paar Physica MCR500) using the analog of an adhesive tack test. The
velocity of the upper plate was controlled and the resulting force load recorded.
The velocities were measured from 0.1 mm/s to 5mm/s.
[0073] The tests were performed on foam samples (Table 6) cut into a 25 mm diameter circle
with a thickness of 0.25 inch. The shear thickening fluid was added to the foam until
the shear thickening response could be felt throughout the foam with pressure applied
between fingers. The amount of shear thickening fluid in the foam composite is less
than the porosity of the foam, and there is air inside of the foam-STF composite.
Table 6 : Polyurethane Based Open Cell Foam Composites
Sample |
Mass Foam (gm) |
Mass STF (gm) |
1 |
0.1773 |
0 |
2 |
0.1743 |
7.3557 |
3 |
0.1771 |
8.4029 |
[0074] Drop Testing: A ¾ inch diameter spherical ball bearing with a mass of 28.81 grams was used for
impact testing. The impact was recorded using a digital camera. Impact testing was
performed on the polyurethane based open cell foams with the compositions given in
Table 6. The spherical ball bearing was dropped onto the target by hand from an approximate
distance of one foot. The results were recorded using a digital camera (Sony P51)
in quick snap mode exposing thirty frames per second.
[0075] Silicone Rubbers: The ability to encapsulate shear thickening fluid inside of the polymer matrix was
studied by mixing quantified amounts of shear thickening fluid with the silicone precursors.
For all silicone rubbers tested the shear thickening fluid was easily mixed into the
precursor. In Figure 7a the discrete regions of STF formed in the silicone can be
observed in a sample 18% mass STF. Figure 7b shows the ribs of shear thickening fluid
formed in Silastic T2 (28% mass STF), highlighted with an arrow, upon curing.
[0076] Figure 7c shows a large droplet of shear thickening fluid (white area) that was contained
inside of a cured Silastic T2 silicone sample. Close observation of the shear thickening
fluid displays cracks due to fracturing of the fluid when the rubber was stretched.
These fissures disappear once the fluid has relaxed. There are many air bubbles contained
within the cured rubber and this problem could be alleviated with the use of a vacuum
chamber prior to curing the sample.
[0077] When the rubber sample shown in Figure 8c is stretched the region of shear thickening
fluid visually changes from translucent to opaque. When fully relaxed the index of
refraction of the shear thickening fluid is closely matched with that of the silicone
and the droplet appears translucent. When a strain is applied to the sample the shear
thickening fluid hardens, and turns into a region of opaqueness. If a large enough
strain is applied the fluid can be torn apart and fissures such as shown above are
readily formed. When the strain is removed the fluid relaxes back to its original
translucent state.
[0078] Open Cell Foam: Shear thickening fluid was added to the open cell foams and found to wet very well.
The shear thickening response of the shear thickening fluid in the open cell foam
is observable by hand in both foams with large and small pores (60-100 parts per inch
(ppi)). The reaction of the material is that of the neat fluid. At fast impact speeds
the foam appears to act like a solid and at low strain rates the composite can be
easily compressed and the fluid is seen to flow out of the pores. This was further
quantified with the impact testing shown later.
[0079] Closed Cell Foam: The addition of shear thickening fluid to the closed cell foam was found to inhibit
the catalyzing agent used to foam and cure the resin. It formed a goop similar in
consistency to baking dough. Even addition of twice as much catalyzing agent to the
mixture does not allow the curing of the foam. We anticipate the successful incorporation
of STF into the closed cell foam through the use of a catalyst that is not inhibited
by the STF.
[0080] Dynamic Testing of Foam STF Composites: Upon slow compression of open cell foams containing STF at stresses between the plates
below critical stress of shear thickening, the fluid was found to flow out of the
foam and collect at the base. Upon retraction of the upper plate to the original position
some of the fluid would be wicked into the foam, however not all was taken into the
pores.
[0081] Dynamic tests were performed on closed cell foam composites with results reported
in
U.S. application Serial Number 11/260,742 filed October 27, 2005, which again is incorporated by reference in its entirety including all the figures..
Analysis of the dynamic properties of a similar shear thickening fluid can be found
in Lee and Wagner (
Lee, Y.S., Wagner, N.J., "Dynamic properties of shear thickening colloidal suspensions,"
Rheol Acta 42,199-208 (2003)). There was a large amount of slip occurring at the plate-foam interface at high
rotation angles. The amount of slip was not quantified but could be eliminated if
the foam were to be attached to the plates using an adhesive. Foam was observed to
fail mechanically at high stresses. However, for the STF-Foam composite there is a
shear thickening effect observed at high stress, such that the
STF-Foam composite was observed to increase in both elasticity and viscosity, in contrast to the Foam itself. It should also be noted that the shear thickening
fluid was observed to thicken visually at these high stress levels.
[0082] Both neat silicone and STF-Silicone composites were tested at various levels of initial
force load. The same problems were encountered for the silicone as in the open cell
foams. There was a tremendous amount of slip present suggesting another testing method
may be more appropriate.
[0083] Compression Testing: The force required to compress open cell foams impregnated with shear thickening
fluid is compared to the compression of the same neat open cell foam. The results
for the neat foam and STF-Foam are shown for the different velocities in Figure 8.
Interestingly the STF modifies the foam response in a somewhat unexpected manner,
allowing the foam to be more "flexible" at low rates while resisting deformation at
higher rates. These results suggested interesting application and as a consequence,
drop impact testing was performed.
[0084] Impact Testing: The response of both neat and impregnated open cell foams was tested against the
impact of a stainless steel ball bearing. The impact of the bearing on the two different
STF-Foam composites was recorded by digital camera as shown in
U.S. application Serial Number 11/260,742 filed October 27, 2005, which again is incorporated by reference in its entirety including all the figures..
The rebound height of the bearing is noticeably subdued for the STF
-foam and almost all of the impact energy is absorbed, a response expected from an
inelastic solid.
[0085] Discussion of Results - The mixing and curing of these rubbers (GE Silicone, Silastic T2, Silastic S) worked
well. When mixed with the shear thickening fluid discrete inclusions of STF could
be formed. This work has shown that containment of shear thickening fluid within a
polymer composite is possible and that the STF remains active.
[0086] Compression measurements and drop testing is conclusive that there is an exploitable
response of the STF-Foam system. There is an obvious visual difference in the impact
of an open cell foam impregnated with shear thickening fluid. This energy absorption
is noticeable and possibly exploitable for practical application. There is an apparent
"hardening" of the foam during the impact as the shear thickening fluid transitions
from its fluid to solid-like state.
[0087] The results of the dynamic rheological tests showed a shear thickening of the viscous
modulus in the open cell foam-STF composite but no clear result of the STF rubber
composites. It is not clear what the actual strain on the sample was because of the
large amount of slip present between the parallel plates and the sample. Further study
would include the use of disposable plates so that the sample could be attached, removing
any possibility of slip.
Example 3 Shear Thickening - Polymer Blends
[0088] Blend Preparation: Suspoemulsions of STFs in polymer melts also show useful and novel properties. The
same PEG-based STF was prepared as stated in the previous two examples and blended
with an ethylene-octene copolymer (Engage® 8200, DuPont-Dow Elastomers). Blending
was performed at 80 °C using a Haake Rheocord with a small counterrotating mixing
chamber at 200 rpm for 2 minutes. The resultant STF-polyolefin was cooled and observed
to be an elastic solid. Samples for tear testing and SEM characterization were prepared
using a Carver press with a custom made die. The compression molding was performed
at 80 °C for 15 minutes melt time and 15000 lbs. compression with 10 minutes annealing.
[0089] The blends were characterized by pressing bars of 45x12x3mm of solid material for
torsional rheological testing and films of 2mm thickness for Trouser Tear Method for
Tear-Propagation Resistance of Plastic Film according to ASTM D 1938. A miniInstron
was employed at a separation rate of 250 mm/min. Scanning Electron Microscopy (SEM)
was performed to characterize the blend morphology by cutting the bars with a razor
blade to expose surface.
[0090] Figure 10 shows micrographs of the phase morphology obtained by scanning electron
microscopy. Clearly the blend morphology has the STF phase dispersed in the matrix
polymer, showing that the blending procedure is successful in producing morphology
with micron-sized features. The STF inclusions are polydisperse with a size that increases
with STF loading. As the STF loading reaches 30% by volume, significant coalescence
of the STF inclusions is observed.
[0091] Mechanical Testing: Torsional rheological testing performed at room temperature using an ARES rheometer
equipped with the torsional testing tooling. All samples showed a higher elastic modulus
than shear modulus, and the modulus is observed to increase with loading of the STF,
with a maximum at 1 vol % of the STF phase. Table 7 shows the elastic modulus at 0.5
% strain amplitude and 1 hz oscillation frequency, showing that the modulus increases
with the addition of STF and then, decreases.
Table 7 Elastic Modulus of Polyolefin STF blends at room terperature measured in torsional
testing at 0.5% strain amplitude as a function of STF phase volume in the blend.
ϕ |
G' at 1 rad/s |
0 |
3.994E6 |
0.010 |
4.094E6 |
0.025 |
3.473E6 |
0.050 |
3.502E6 |
0.100 |
3.147E6 |
0.300 |
2.964E6 |
[0092] Tear Testing: Representative tear test (ASTM D 1938 standard) results are shown in Figure 10, where
it is seen that neat polyolefin and 1 vol% STF blends tear through, whereas 2.5, 5,
10 and 20 vol % do not fail during the course of the experiment.
[0093] Higher STF loadings lead to failure. Clearly, the presence of STF inclusions in the
PO leads to substantial enhancement in tear resistance.
[0094] Table 8 shows the energy dissipated during the tear testing, which is defined as
the area under the force versus displacement curves shown in Figure 12.
Table 8 Energy dissipation during Tear Tests.
Volume Fraction of STF Phase |
Energy Dissipated During Tear Test (J) |
0.01 |
0.64 |
0.025 |
0.83 |
0.05 |
0.97 |
0.1 |
0.89 |
0.2 |
0.67 |
0.3 |
0.06 |
0.01 |
0.64 |
0.025 |
0.83 |
[0095] Blends with 5 vol% STF dissipate the highest energy during this test, and this value
is more than 3 times higher than the neat polyolefin.
[0096] Table 9 displays the maximum load achieved and the maximum extension.
Table 9 Tear Testing Results for Maximum Load and Maximum Extension
Volume Fraction of STF Phase |
Maximum Load (N) |
Extension at Maximum Load (m) |
0.01 |
5.3 |
0.1568 |
0.025 |
5.4 |
0.2033 |
0.05 |
6.3 |
0.2073 |
0.1 |
5.9 |
0.2068 |
0.2 |
4.6 |
0.208 |
0.3 |
1.8 |
0.0642 |
[0097] The latter is limited by the instrument to 0.2 meters, and so the true maximum extension
achievable is at least this high if not higher for the STF-PO blends that do not fracture.
The maximum load achieved is for the 5 vol% STF-PO blend, and the maximum extension
is reached already for the 2.5 vol% blend. The presence of STF inclusions increases
the maximum load by over 20% and the maximum extension to >75% above the performance
of the neat PO.
[0098] ASTM D 882 tensile elongation tests were performed using the miniInstron with serrated
grip faces at an initial grip separation of 50 mm and an elongation rate of 250 mm/min.
The results, shown in Table 10 shows that the 1 vol% STF-PO blend has the highest
shear modulus and the 10% STF-PO blend has the Young's modulus.
Table 10 Elastic Shear Modulus G' and Young's Modulus of STF-PO blends.
Volume Fraction of STF Phase |
G' (100 Rad/s) (MPa) |
Young's Modulus (MPa) |
0 |
4.7 |
|
0.01 |
4.78 |
|
0.025 |
4.33 |
|
0.05 |
3.87 |
6.07 |
0.1 |
3.5 |
7.11 |
0.2 |
3.65 |
5.24 |
0.3 |
3.45 |
|
[0099] Further, the neat PO, not shown, fails at the grip and does not extend. This is further
evidence of the strengthening and toughening effect of the STF inclusions during deformation
of the PO blends.
SUMMARY
[0100] We have demonstrated a novel composite material comprised of a shear thickening fluid
(STF) blended or integrated with an immiscible or largely immiscible matrix material.
As shown, the matrix material can be a liquid, thus forming a novel suspoemulsion
where one phase is shear thickening, which imparts novel rheological behavior. This
behavior is a consequence of the STF material's transition at high stresses or shear
rates, but also is shown to depend on the state of dispersion of the STF phase. The
novel rheology includes a stiffening along with increased rate of energy dissipation
at high stresses or shear rates, as observed in both steady, transient, and dynamic
oscillatory experiments. The matrix material can also be a foam or crosslinked rubber,
where the STF phase is dispersed therein or vice versa. The formation of this composite
can be formed by reactive polymerization or crosslinking in the non-STF phase. The
matrix material can also be a thermoplastic where the STF is mechanically or otherwise
blended into the matrix material while the matrix material is in the molten state,
which then solidified upon cooling to the temperature of use. Such novel materials
exhibit unique dynamic behavior and show enhancement in mechanical properties and
energy absorption upon mechanical impact, as well as enhanced tear resistance and
elongational strength.
[0101] All the references described above are incorporated by reference in its entirety
for all useful purposes.
[0102] While there is shown and described certain specific structures embodying the invention,
it will be manifest to those skilled in the art that various modifications and rearrangements
of the parts may be made without departing from the spirit and scope of the underlying
inventive concept and that the same is not limited to the particular forms herein
shown and described.